Distributed distillation has been suggested as a basis for the design of refinery systems, ethylene recovery systems, and other commercial chemical, petroleum and petrochemical separations systems operations for many years. Distributed distillation is best understood by contrasting it with sharp split distillation. In sharp split distillation, a separation is made between light and heavy components that are adjacent to each other on the volatility curve of the mixture being separated. That is, there are no compounds in the mixture that have volatility that is intermediate to those of the light and heavy components.
For example, a typical sharp-split deethanizer column in an ethylene recovery system performs a sharp split between ethane and propylene. The overheads of the column contain essentially no propylene and the bottoms contain essentially no ethane. The overheads therefore contain all components lighter than the light key component (e.g. ethylene, methane, etc.), and the bottoms contain all components heavier than the heavy component (e.g. propane, C4s, etc).
In a distributed distillation operation, a sharp split is not made between components that are adjacent on the volatility curve. A distributed distillation analog to the deethanizer is a “C2s distributor”. A C2s distributor column produces a sharp split between methane and C3 components while distributing ethane and ethylene between the column overhead and bottoms. In a C2s distributor column, the light component is methane and the heavy component is propylene. These components are not adjacent to each other on the volatility curve; ethane and ethylene have a volatility that is intermediate between methane and propylene. In this case, then, ethane and ethylene “distribute” between the column overheads and bottoms. The overheads contain some ethane and ethylene, as well as methane and lighter components, but essentially no propylene. The bottoms also contain some ethane and ethylene, as well as propylene and heavier components, but essentially no methane. Of course, further purification of the components must be done in downstream columns.
A benefit of a distributed distillation system is that it requires less total energy to produce the final purified components than an analogous “sharp split” distillation sequence. A way of understanding the energy savings provided by distributed distillation is that it accomplishes the separation of components with fewer overall phase changes. Phase changes (condensation or vaporization) require energy, and reducing the number of phase changes also reduces the energy consumption of the system.
Thermal coupling of columns is another method for improving the overall energy efficiency of a distillation-based system. Thermal coupling of columns consists of providing liquid reflux to a column with a liquid side draw from a downstream column, or providing stripping vapor to a column with a vapor side draw from a downstream column. In this way, the composition of the reflux liquid at the top of the column is much closer to the equilibrium composition existing at the top of the column than could be produced by a typical condenser. Likewise, the composition of the stripping vapor sent to the bottom of the column is much closer to the equilibrium composition existing at the bottom of the column than if the vapor were generated with a conventional reboiler. This thermal coupling reduces the amounts of heavier components “remixed” into the bottom of the column and the amount of lighter components “remixed” into the top of the column. This improves the thermodynamic efficiency of the column, reducing reflux and reboil rates and thereby saving energy.
In addition, dividing wall columns have been recited in the prior art as a way to combine distinct distillation processes within a single pressure shell. Wright (U.S. Pat. No. 2,471,134) disclosed a partitioned fractionating column for separating components of a composite fluid in 1949. Petlyuk et al. (Int. Chem. Eng. 5, pp 555–561, 1965) disclosed a systematic discussion of dividing wall columns in 1965. The early dividing wall column designs included a dividing wall within the middle section of a column, with open, full-diameter rectifying and stripping sections in the top and bottom of the column, respectively. Oginsy (U.S. Pat. No. 5,709,780) disclosed a dividing wall column in which the vertical dividing wall extends from the middle section of the column all the way to the bottom of the column. The dividing wall therefore supplied the column with two stripping or absorption sections. Oginsy (U.S. Pat. No. 5,755,933) further disclosed a dividing wall column in which the vertical dividing wall extends from the middle section of the column all the way to the top of the column. The dividing wall in this case supplied the column with two separate rectifying sections.
Stork (U.S. Pat. No. 6,077,985) discloses the use of a dividing wall column for combining the deethanizing and deethyleneizing functions of an olefins plant separation train. This column design contains a dividing wall that extends from the middle section of a column all the way to the bottom of the column. The column is therefore split into two bottoms sections. Feed enters to the middle of one of these sections, which acts as a deethanizer column. The full-diameter rectifying section of the column acts as an ethylene rectifying section, and the section on the other side of the dividing wall acts as an ethylene stripper. However, both of the distillation functions disclosed by Stork utilize sharp-splits rather than distributed distillation.
Manley (U.S. Pat. No. 5,675,054) discloses the combined use of C2s distributors and ethylene distributors for the recovery of ethylene from cracked gas mixtures for a variety of feed types. Manley and Hahesy (Hydrocarbon Processing, April 1999, p 117) teaches combining separate rectifying sections into a single shell, and combining separate stripping sections into a single shell, but not combining rectifying and stripping sections from two separate distributed distillation columns into a single shell.
The present invention relates to a distillation column in which two distinct thermally coupled distributed distillation functions for ethylene recovery and purification are combined into a single shell. A separating wall within the single shell divides the column into zones in which the distinct distillation functions take place. This invention presents an apparatus that allows for a capital-efficient implementation of distributed distillation in the recovery and partial purification of ethylene. This invention further relates to the recovery of ethylene from a cracked gas stream through the use of distributed distillation, particularly through the combined use of a C2s distributor and an ethylene distributor.
Prior art designs utilize separate distillation columns for the C2s distributor and ethylene distributor functions. Two separate pressure vessels, foundations, and support structures are required to build and operate the prior art system. This invention is an improvement over the prior art in that both distillation functions are combined into a single pressure shell. Therefore, only one pressure vessel, foundation, and support structure is required using the design of this invention. This results in a significant reduction in both materials and installation costs compared with the prior art.